DESCRIPTION

The
open()
system call is used to convert a pathname into a file descriptor
(a small, non-negative integer for use in subsequent I/O as with
read, write, etc.).
When the call is successful, the file descriptor returned will be
the lowest file descriptor not currently open for the process.
This call creates a new open file, not shared with any other process.
(But shared open files may arise via the
fork(2)
system call.)
The new file descriptor is set to remain open across exec functions
(see
fcntl(2)).
The file offset is set to the beginning of the file.

The parameter
flags
is one of
O_RDONLY, O_WRONLY or O_RDWR
which request opening the file read-only, write-only or read/write,
respectively,
bitwise-or'd
with zero or more of the following:

O_CREAT

If the file does not exist it will be created.
The owner (user ID) of the file is set to the effective user ID
of the process. The group ownership (group ID) is set either to
the effective group ID of the process or to the group ID of the
parent directory (depending on filesystem type and mount options,
and the mode of the parent directory, see, e.g., the mount options
bsdgroups
and
sysvgroups
of the ext2 filesystem, as described in
mount(8)).

O_EXCL

When used with
O_CREAT,
if the file already exists it is an error and the
open
will fail. In this context, a symbolic link exists, regardless
of where its points to.
O_EXCL
is broken on NFS file systems, programs which rely on it for performing
locking tasks will contain a race condition. The solution for performing
atomic file locking using a lockfile is to create a unique file on the same
fs (e.g., incorporating hostname and pid), use
link(2)
to make a link to the lockfile. If link() returns 0, the lock is
successful. Otherwise, use
stat(2)
on the unique file to check if its link count has increased to 2,
in which case the lock is also successful.

O_NOCTTY

If
pathname
refers to a terminal device --- see
tty(4)
--- it will not become the process's controlling terminal even if the
process does not have one.

O_TRUNC

If the file already exists and is a regular file and the open mode allows
writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0.
If the file is a FIFO or terminal device file, the O_TRUNC
flag is ignored. Otherwise the effect of O_TRUNC is unspecified.

O_APPEND

The file is opened in append mode. Before each
write,
the file pointer is positioned at the end of the file,
as if with
lseek.
O_APPEND
may lead to corrupted files on NFS file systems if more than one process
appends data to a file at once. This is because NFS does not support
appending to a file, so the client kernel has to simulate it, which
can't be done without a race condition.

O_NONBLOCK or O_NDELAY

When possible, the file is opened in non-blocking mode. Neither the
open
nor any subsequent operations on the file descriptor which is
returned will cause the calling process to wait.
For the handling of FIFOs (named pipes), see also
fifo(4).
This mode need not have any effect on files other than FIFOs.

O_SYNC

The file is opened for synchronous I/O. Any
writes
on the resulting file descriptor will block the calling process until
the data has been physically written to the underlying hardware.
See RESTRICTIONS below, though.

O_NOFOLLOW

If pathname is a symbolic link, then the open fails. This is a
FreeBSD extension, which was added to Linux in version 2.1.126.
Symbolic links in earlier components of the pathname will still be
followed. The headers from glibc 2.0.100 and later include a
definition of this flag; kernels before 2.1.126 will ignore it if
used.

O_DIRECTORY

If pathname is not a directory, cause the open to fail. This
flag is Linux-specific, and was added in kernel version 2.1.126, to
avoid denial-of-service problems if opendir(3) is called on a
FIFO or tape device, but should not be used outside of the
implementation of opendir.

O_DIRECT

Try to minimize cache effects of the I/O to and from this file.
In general this will degrade performance, but it is useful in
special situations, such as when applications do their own caching.
File I/O is done directly to/from user space buffers.
The I/O is synchronous, i.e., at the completion of the
read(2)
or
write(2)
system call, data is guaranteed to have been transferred.
Under Linux 2.4 transfer sizes, and the alignment of user buffer
and file offset must all be multiples of the logical block size
of the file system. Under Linux 2.6 alignment to 512-byte boundaries
suffices.
A semantically similar interface for block devices is described in
raw(8).

O_ASYNC

Generate a signal (SIGIO by default, but this can be changed via
fcntl(2))
when input or output becomes possible on this file descriptor.
This feature is only available for terminals, pseudo-terminals, and
sockets. See
fcntl(2)
for further details.

O_LARGEFILE

On 32-bit systems that support the Large Files System, allow files
whose sizes cannot be represented in 31 bits to be opened.

Some of these optional flags can be altered using
fcntl
after the file has been opened.

The argument
mode
specifies the permissions to use in case a new file is created. It is
modified by the process's
umask
in the usual way: the permissions of the created file are
(mode & ~umask).
Note that this mode only applies to future accesses of the
newly created file; the
open
call that creates a read-only file may well return a read/write
file descriptor.

The following symbolic constants are provided for
mode:

S_IRWXU

00700 user (file owner) has read, write and execute permission

S_IRUSR (S_IREAD)

00400 user has read permission

S_IWUSR (S_IWRITE)

00200 user has write permission

S_IXUSR (S_IEXEC)

00100 user has execute permission

S_IRWXG

00070 group has read, write and execute permission

S_IRGRP

00040 group has read permission

S_IWGRP

00020 group has write permission

S_IXGRP

00010 group has execute permission

S_IRWXO

00007 others have read, write and execute permission

S_IROTH

00004 others have read permission

S_IWOTH

00002 others have write permisson

S_IXOTH

00001 others have execute permission

mode
must be specified when
O_CREAT
is in the
flags,
and is ignored otherwise.

creat
is equivalent to
open
with
flags
equal to
O_CREAT|O_WRONLY|O_TRUNC.

RETURN VALUE

open and creat
return the new file descriptor, or -1 if an error occurred (in which case,
errno
is set appropriately).
Note that
open
can open device special files, but
creat
cannot create them - use
mknod(2)
instead.

On NFS file systems with UID mapping enabled, open may return a file
descriptor but e.g. read(2) requests are denied with EACCES.
This is because the client performs open by checking the permissions,
but UID mapping is performed by the server upon read and write requests.

If the file is newly created, its atime, ctime, mtime fields are set
to the current time, and so are the ctime and mtime fields of the
parent directory.
Otherwise, if the file is modified because of the O_TRUNC flag,
its ctime and mtime fields are set to the current time.

ERRORS

EEXIST

pathname
already exists and
O_CREAT and O_EXCL
were used.

EISDIR

pathname
refers to a directory and the access requested involved writing
(that is,
O_WRONLY
or
O_RDWR
is set).

EACCES

The requested access to the file is not allowed, or one of the
directories in
pathname
did not allow search (execute) permission, or the file did not exist
yet and write access to the parent directory is not allowed.

ENAMETOOLONG

pathname was too long.

ENOENT

O_CREAT is not set and the named file does not exist.
Or, a directory component in
pathname
does not exist or is a dangling symbolic link.

ENOTDIR

A component used as a directory in
pathname
is not, in fact, a directory, or O_DIRECTORY was specified and
pathname
was not a directory.

ENXIO

O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and
no process has the file open for reading.
Or, the file is a device special file and no corresponding device exists.

ENODEV

pathname
refers to a device special file and no corresponding device exists.
(This is a Linux kernel bug - in this situation ENXIO must be returned.)

EROFS

pathname
refers to a file on a read-only filesystem and write access was
requested.

ETXTBSY

pathname
refers to an executable image which is currently being executed and
write access was requested.

EFAULT

pathname points outside your accessible address space.

ELOOP

Too many symbolic links were encountered in resolving
pathname,
or O_NOFOLLOW was specified but
pathname
was a symbolic link.

ENOSPC

pathname
was to be created but the device containing
pathname
has no room for the new file.

ENOMEM

Insufficient kernel memory was available.

EMFILE

The process already has the maximum number of files open.

ENFILE

The limit on the total number of files open on the system has been
reached.

NOTE

Under Linux, the O_NONBLOCK flag indicates that one wants to open
but does not necessarily have the intention to read or write.
This is typically used to open devices in order to get a file descriptor
for use with
ioctl(2).

CONFORMING TO

SVr4, SVID, POSIX, X/OPEN, BSD 4.3.
The
O_NOFOLLOW
and
O_DIRECTORY
flags are Linux-specific.
One may have to define the
_GNU_SOURCE
macro to get their definitions.

The (undefined) effect of
O_RDONLY | O_TRUNC
various among implementations. On many systems the file is actually
truncated.

The
O_DIRECT
flag was introduced in SGI IRIX, where it has alignment restrictions
similar to those of Linux 2.4. IRIX has also a fcntl(2) call to
query appropriate alignments, and sizes. FreeBSD 4.x introduced
a flag of same name, but without alignment restrictions.
Support was added under Linux in kernel version 2.4.10.
Older Linux kernels simply ignore this flag.

BUGS

"The thing that has always disturbed me about O_DIRECT is that the whole
interface is just stupid, and was probably designed by a deranged monkey
on some serious mind-controlling substances." -- Linus

RESTRICTIONS

There are many infelicities in the protocol underlying NFS, affecting
amongst others
O_SYNC and O_NDELAY.

POSIX provides for three different variants of synchronised I/O,
corresponding to the flags O_SYNC, O_DSYNC and
O_RSYNC. Currently (2.1.130) these are all synonymous under Linux.